1. Field of the Invention
The invention relates to the production of aluminum by igneous electrolysis using the Hall-Heroult process and installations designed for industrial use of this process. More specifically, the invention relates to control of thermal fluxes in electrolytic pots and cooling means for obtaining this control.
2. Description of the Related Art
Metal aluminum is produced industrially by igneous electrolysis, namely electrolysis of alumina in solution in a molten cryolite bath called an electrolyte bath, using the well known Hall-Heroult process. The electrolyte bath is contained in a pot comprising a steel pot shell which is coated on the inside with refractory and/or insulating materials, and a cathodic assembly located at the bottom of the pot. The electrolysis current, which may reach values of more than 300 kA, generates alumina reduction reactions and also keeps the electrolyte bath at a temperature of the order of 950.degree. C. by the Joule effect.
The electrolytic pot is usually controlled such that it is in thermal equilibrium. In other words, the total amount of heat dissipated by the electrolytic pot is compensated by heat produced in the pot, which is essentially derived from the electrolysis current. The thermal equilibrium point is usually chosen to give the best operating conditions, both technically and economically. In particular, the possibility of maintaining an optimum set temperature results in a significant saving in the aluminum production cost because the Faraday efficiency is kept at a very high value, exceeding 90% in the most efficient plants.
Thermal equilibrium conditions depend on the physical parameters of the pot such as the dimensions and nature of the component materials, and pot operating conditions such as the electrical resistance of the pot, the bath temperature or the intensity of the electrolysis current. The pot is frequently constructed and controlled such that a ridge of solidified bath is formed on the side walls of the pot, which in particular prevents the lining of the walls being attacked by the liquid cryolite.
The igneous electrolysis aluminum production industry is regularly confronted with the need for industrial installations that stabilize and maintain the operating point of electrolytic pots for the purposes of optimized plant management, but they must also accept deliberate changes to operating conditions which may be quite different from normal conditions. In other words, it is frequently useful to be able to control or modulate easily the operating point of plant electrolytic pots while maintaining or even improving their normal technical performances, without correspondingly increasing production costs. For example, this type of situation arises when it is required to vary the power of a series of electrolytic pots (i.e. a potline) as a function of an electricity contract.
With this objective in mind, the applicants looked for methods and means of controlling thermal fluxes and stabilizing the thermal conditions of electrolytic pots, which do not require a high investment and do not involve unacceptable additional operating costs, while providing very good efficiency and adaptability.
It has already been suggested that pots should be equipped with specific means for evacuating and dissipating heat produced in a controlled manner. In particular, the Soviet Invention Certificate Nos. SU 605,865 and SU 663,760 suggest providing pots with a cooling system controlled from the outside. Such a pot includes hermetic cavities on the sides, variable thermal shields and air conveyer means equipped with regulation valves below the pot. Air is discharged in air conveyer means through a fan or a compressor. These devices require a large and cumbersome infrastructure.
European Patent Application Serial No. 47,227 has also suggested that the thermal insulation on the pot should be increased and that the pot should be provided with heat ducts equipped with heat exchangers. Heat ducts pass through the pot shell and the thermal insulation and are plugged into carbonaceous parts such as edge slabs. This solution is fairly complex, expensive to install, and also requires major modifications to the pot.
In order to more specifically encourage the formation of a ridge of solidified bath, U.S. Pat. No. 4,087,345 also describes a pot shell equipped with stiffeners and a reinforcement frame constructed to encourage cooling of the pot sides by natural convection of ambient air. This type of device requires installations fixed to the pot shell. Furthermore, static devices are not very suitable for precise control of thermal fluxes.
In order to control the formation of the ridge of solidified bath and to recover part of the heat drawn out from the sides of the pot, U.S. Pat. No. 4,608,135 proposes using a pot in which passages are laid out between the edge slabs and the inside insulation of the pot shell, and air inlet orifices on the sides of the pot. Passages communicate firstly with the orifices and secondly with the inside of the collection device fixed on the pot. The collection device draws in ambient air taken from the sides of the pot through the orifices and entrains its flow through the passages along the edge slabs. This device has the effect of cooling them. The air flow is controlled by dampers fitted with valves located on the sides of the collection device, which valves act as bypass pipes. This device requires major modifications to the pot and does not enable independent control of cooling, since regular work on the pot makes it necessary to open the collection device covers and to disturb the effect of the dampers.
After observing that there are no satisfactory known solutions, the applicants set the objective of finding efficient and adaptable means of evacuating and dissipating heat produced by the electrolytic pot, so that the pot can easily be set up and which does not require any major modifications to the pot, particularly to the pot shell, or to any major infrastructure. In order to enable use in existing plants and in new plants in particular, the applicants searched for means of modifying the power of the pots, which modifiers can be easily adapted to different types of pots or to different operating modes of the same type of pot, and which are suitable for industrial installations comprising a large number of pots in series.